Women, who comprise about one third of cocaine addicts, start using cocaine earlier in life, are more sensitive than men to some cocaine effects and progress to dependence more rapidly. Rats show some similarities to this pattern: cocaine elicits greater increases in cocaine-stimulated locomotion, females work harder for cocaine reinforcement and extracellular DA rises more than in male rats. One previously uninvestigated mechanism for this sex difference in dopaminergic function is sex-specific trophic actions which maintain dopamine neurons. We have shown that removal of gonadal steroids has marked and opposite effects on DA neuron number in the substantia nigra/ventral tegmental area in male and female rats. Ovariectomy causes a significant fall in DA neuron number, while castration of males results in a surprising increase in cell number. We hypothesize that dopamine neuron number is differentially regulated in males and females: while in males, androgens suppress neurogenesis/permit cell death, in females, estrogen enhances cell survival/neurogenesis. The purpose of this proposal is to investigate the endocrine and cellular mediators of gonadal steroid regulation of dopamine neuron number. In addition, we will investigate the association between cell loss, dopaminergic function and the behavioral effects of cocaine. To investigate this hypothesis we will pursue three specific aims. (1) Evaluate the ability of gonadal steroids to prevent change in DA neuron number, dopaminergic function and locomotor stimulant effects of cocaine after gonadectomy (GDX). The ability of estrogen receptor agonists to maintain these parameters in ovariectomized (OVX) females and castrated (CAS) males, and of androgen to do the same in CAS males will be determined. This first aim will be accomplished by using the novel strategy of evaluating behavioral response to cocaine, DA release and DA cell number in the same animal. (2) Determine if the change in DA neuron number after GDX reflects accelerated cell death in females/retarded cell death in males. Measures of cell death including TUNEL and specific cell death cascade mediators will be conducted at intervals after surgery and hormone replacement in sham and GDX males and females. (3) We will investigate if the gain in DA neurons after castration results from addition of TH-positive cells and will evaluate neurogenesis in DA neurons in sham and CAS males and females using BRDU incorporation and markers for newly differentiated neurons. These results will have significant implications understanding both sex differences in substance abuse vulnerability and Parkinson's Disease. PUBLIC HEALTH RELEVANCE: The purpose of this project is to understand what biologic factors influence the development and progression of stimulant addiction in females. We plan to investigate in rats the hypothesis that estrogen maintains dopamine cell number and innervation density and so enhances cocaine-stimulated dopamine overflow and the behavioral response to cocaine. In addition, we will determine if androgens like testosterone may exert the opposite effect to limit dopamine cell number, and so to limit cocaine-stimulated dopamine overflow and behavior. To accomplish these aims, we will measure cocaine-stimulated behavior and dopamine overflow in male and female rats that are gonadally intact, gonadectomized or hormone replaced, and in the same animals will quantitate dopamine cell number and innervation density of areas critical for cocaine-stimulated behavior.